Instantaneous transmission systems are arranged such that a new gear can be selected whilst the current gear is still engaged. When the new gear has been selected the initial gear is released. Thus during a shift, for at least one shift type, torque is substantially continuously supplied to the output of the transmission. Hence such shifts are said to be instantaneous because there is no delay in selecting the new gear whereas for conventional transmissions it is necessary to deselect the current gear, move through a neutral phase, and then select the new gear.
In some known instantaneous type transmission systems, such as those described in WO 2004/099654. WO 2005/005868, WO 2005/005869, WO 2005/024261 and WO 2005/026570 shift conflicts and shift control problems can occur under certain circumstances that can lead to failure of the transmission.
The known transmissions mentioned above have a plurality of gear trains for transmitting drive between transmission input and output shafts. For a first gear train, a first gear wheel is rotatably mounted on either a transmission input shaft or an output shaft and a second gear wheel is fixed to the other shaft, in mesh with the first gear wheel. A second gear train comprising third and fourth gear wheels is similarly arranged. The transmission also includes at least one gear selector mechanism that is located between the rotatably mounted gear wheels that is arranged to selectively lock them for rotation with the shaft on which they are mounted. When a gear wheel from a gear train is locked for rotation with the shaft, drive is transmitted between the input and output shafts via that gear train.
The arrangement of the transmission is such that when drive is transmitted between the input and output shafts via one of the gear trains the gear selector mechanism can select a new gear train under power without first disengaging the first gear train, by locking the rotatably mounted gear wheel of the second gear train to its shaft. Thus momentarily, drive is transmitted between the input and output shaft via two gear trains simultaneously. The new gear train then overdrives the first gear train and the selector mechanism disengages the first gear wheel. Drive is then transmitted between the input and output shafts via the new gear train only. Since it is not necessary to open the clutch when changing gear the transmission provides uninterrupted power through a gearshift.
An instantaneous gear selector mechanism typically has four modes of operation with respect to each of the rotatably mounted gear wheels associated with it:                Fully engaged in both torque directions (fully in gear);        Disengaged in both torque directions (neutral);        Engaged in the forward torque direction while disengaged in the reverse torque direction;        Disengaged in the forward toque direction while engaged in the reverse torque direction.        
The last two modes enable a discrete ratio gearbox to have the ability to shift up or down ratios instantly under load without torque interruption. In some embodiments it is not necessary to have a neutral position.
However, there is an inherent failure mode in known transmissions including a plurality of selector assemblies and having the last two modes of operation where it is necessary to operate both selector assemblies in order to select a new gear. That is, it is possible for two gears to be engaged simultaneously with opposing torque directions under some conditions, which can cause the transmission to lock up or the engagement members to ramp out, depending on the type of shift.
For example, a transmission system typically includes at least three gear trains, and is likely to include four to six gear trains. A transmission having four gear trains and a conventional layout with all the selector mechanisms mounted on the same shaft, typically requires two instantaneous gear selector mechanisms. The first gear selector mechanism is arranged to selectively engage the first and second gear trains and the second gear selector mechanism is arranged to selectively engage the third and fourth gear trains. Each gear selector mechanism includes first and second sets of engagement members having opposed ends with fixed opposed directions of torque transfer. This provides an inherent fail-safe arrangement against the above mentioned failure mode where the shift is from a gear on one side of the selector mechanism to a gear on the other side of the same selector mechanism, for example when the first selector mechanism selects between the first and second gears or when the second selector mechanism selects between the third and fourth gears.
The failure mode described above can only occur if a gearshift is from a gear that is engageable by one of the gear selector mechanisms to a gear that is engageable by the other gear selector mechanism, for example when changing between second and third gears in the four speed transmission mentioned above, since this requires movement of both the first and second gear selector mechanisms.
Most of the known instantaneous gear selector mechanisms include an actuator assembly for controlling operation of the engagement members that includes a so called “dual fork” arrangement. That is both sets of engagement members are controlled by a single fork having a first part for controlling the movement of the first set and a second part (dual) for controlling the movement of the second set of members. Thus a single actuation affects the movement/operation of both sets of engagement members while allowing some independent movement of the first and second sets of engagement members. For example, a single actuation may cause one set to be moved out of engagement with a gear wheel whilst the other set remains engaged with the gear wheel. The inventors have found that this arrangement can be a significant cause of gear selection conflicts that occur when selecting a new gear requires movement of more than one gear selector mechanism.
WO 2006/123128 addresses this problem by having a transmission system including a blocking mechanism to prevent certain conflict shifts occurring, however the system disclosed adds significant weight and inertia to the transmission.
The transmission system disclosed in WO 2005/026570 can theoretically be used to address the problem however in practice electromagnetically actuated gear selector mechanisms are very difficult to implement since there are difficulties in controlling operation of the magnetic fields in order to obtain the degree of control required for the reliable engagement and disengagement for a successful gearshift. Furthermore, the range of gearshift types required for a working transmission system go beyond the control abilities of this type of transmission.
Another problem with the “dual fork” type of instantaneous gear selector mechanisms described in WO 2004/099654, WO 2005/005868, WO 2005/005869 and WO 2005/024261 is that when a transmission operates at low speeds the biasing force that is applied to the drivingly engaged set of engagement members by the actuator via the disc spring during a shift may be greater than the frictional force between the engagement members and the dogs. If this arises, the engagement members can accidentally disengage from the current gear wheel before the new gear wheel is engaged, leading to a loss of drive. This is unacceptable for automotive applications.
A further problem with the known instantaneous systems is that when performing kick-down shifts (accelerating down shifts) in a vehicle, there is a tendency for the vehicle to lurch when the new gear is engaged. This leads to a very uncomfortable ride for the occupants of the vehicle. A further problem arises with kick-down shifts because the newly engaged gear wheel cannot overdrive the former gear wheel due to the differences in gear wheel geometry. When addressing this problem, previous systems used the approach of fully disengaging the current gear train before engaging the new gear train. However with this approach there is a substantial interruption in power transfer that is detectable by the driver and so the method is not optimised. Therefore a method of performing a kick-down shift without substantial power interruption (that is without a power interruption that is noticeable by the driver of a vehicle) is highly desirable. This is a very important problem to be solved for instantaneous transmission systems since it is necessary to be able to perform kick-down shifts in an acceptable manner in order for this type of transmission system to be suitable for road vehicles.
Accordingly the present invention seeks to provide a transmission system, a drive system, a transmission control system and gear selection methods, that mitigate at least one of the aforementioned problems.